1. Technical Field
The present invention is directed to a heterostructure Field-Effect Transistor (“FET”). More particularly, the present invention is directed to a heterostructure FET having a superlattice structure with a plurality of Two-Dimensional Electron Gas (2DEG) channels, all of which are capable of being turned “off” by the application of a relatively low gate voltage. The present invention is equally applicable to superlattice structures having a plurality of Two-Dimensional Hole Gas (2DHG) channels. The present invention finds particular utility as a normally-on RF switch.
2. Background Information
A high-quality RF switch is ideally designed to minimize the on-resistance, Ron, while also minimizing the off-capacitance, Coff. An ideal RF switch, when “on”, passes a signal without attenuation, distortion or insertion loss (all of which being a function of on-resistance) and, when “off”, isolates the signal and prevents it from leaking through the switch (leakage being a function of the off-capacitance).
In order to be able to use an RF switch in as many applications as possible, it is desirable that the RF switch has a large bandwidth, which is proportional to the Figure of Merit for RF switches, the RF switch cut-off frequency. The formula for the RF switch cut-off frequency FCO is as follows:FCO=1/(2πRonCoff)  [Equation 1].
In conventional FET designs, any attempt to lower the on-resistance, such as by increasing the periphery of the device, generally causes the off-capacitance to increase proportionately. Given the inversely-proportional relationship between on-resistance and off-capacitance, it is difficult to improve (e.g., lower) insertion loss while not adversely impacting (e.g., lowering) the isolation simultaneously.
The prior art has attempted to reduce on-resistance by fabricating, in a laboratory setting, heterostructure FETs having a plurality of 2DEG channels. The term “heterostructure” (also, “hetero-junction”) refers to a structure having two distinct layers of dissimilar material in intimate contact with each other. A superlattice structure is formed by manufacturing a plurality of periodically-repeated heterojunctions one on top of the other in a stacked relationship.
Certain heterostructure materials, such as Aluminum Gallium Nitride (AlGaN) and Gallium Nitride (GaN), create an electron well (i.e., a sheet of electrons) at the interface between the two dissimilar materials resulting from the piezoelectric effect and spontaneous polarization effect therebetween. The resulting sheet of electrons that forms at this interface is typically referred to as a Two-Dimensional Electron Gas (“2DEG”) channel. FETs that operate by generating and controlling the electrons in the 2DEG channel are conventionally referred to as high electron mobility transistors (“HEMTs”).
By stacking a plurality of these two-material heterostructures, and with the addition of appropriate doping in the layers to maintain the presence of the 2DEG channels when stacking a plurality of heterostructure layers, the electron sheets are able to act in parallel, allowing for greater current flow through the superlattice device.
When this type of FET is “on”, the superlattice device has a lower on-resistance, relative to a single heterostructure-layer device, because the multiple 2DEG channels allow a proportionally higher current to flow between the source and drain, resulting in an overall reduction in on-resistance.
Coincidentally, the plurality of stacked heterostructure layers does not increase the off-capacitance proportional to the number of stacked layers. On-resistance is a two-dimensional measurement (i.e., between the source and drain), while off-capacitance is a three-dimensional measurement: off-state blocking voltages create three-dimensional fringing fields having a fringing-field capacitance and forming a significant portion of the off-state capacitance, which is a function of the spacing between the source and drain, as well as the volume and type of materials therebetween. Where the heterostructure layers are relatively thin, the volumetric material between source and drain is not proportionally increased. Thus, a thin-film, stacked heterostructure configuration decreases the on-resistance without proportionally impacting the off-capacitance, thereby allowing for a higher cut-off frequency and thus a larger bandwidth.
While the prior art has been able to fabricate, in a laboratory setting, stacked heterostructure FETs, the prior art devices exhibit excessively large leakage currents and are unable to be turned completely “off”, even with the application of relatively high gate voltages, rendering such devices to be of limited utility. See, e.g., T. Palacios, et al., “Use of Double-Channel Heterostructures to Improve the Access Resistance and Linearity in GaN-Based HEMTs”, IEEE Transactions on Electron Devices, Vol. 53, No. 3, pgs. 562-65 (March 2006) [Reference 1]; R. Chu, “AlGaN—GaN Double-Channel HEMTs”, IEEE Transactions on Electron Devices, Vol. 52, No. 4, pgs. 438-46 (April 2005) [Reference 2]; S. Heikman, et al., “High conductivity modulation doped AlGaN/GaN multiple channel heterostructures”, J. of Applied Physics, Vol. 94, No. 8, pgs. 5321-25 (Oct. 15, 2003) [Reference 3]; and N. H. Sheng, et al., “Multiple-Channel GaAs/AlGaAs High Electron Mobility Transistors”, IEEE Electron Device Letters, Vol. EDL-6, No. 6, pgs. 307-10 (June 1985) [Reference 4], all of which are incorporated herein by reference.